In Blender often the term Shading or Shader is used. I still don't understand this terminology very well. Yes I have tried and for instance read the manual but I think it is because english is not my native language that I still don't understand the terminology very well. Can someone explain what is meant with Shading and/or a Shader ?
The term Shading comes from the pre-3D computer era - I guess it spread in Renaissance when art was concerned with how reality looks like.
Shading (in 2D) is the act of adding shade and lit areas to drawn objects, to give them volume and to capture the look of material.
Shader is a (sub-)program that does shading. Nowadays we have math models that model the light interaction with the material - for different materials there are different math models to keep things optimized and fast. Each shader then implements different math model and is responsible for how the surface is rendered.
The act of building and combining shaders is called shader building (not shading), the act of creating textures for the shaders is called texturing, and the act of UV (or other) mapping with texturing and shader building is called surfacing.
Flat shading is a lighting technique used in 3D computer graphics to shade each polygon of an object based on the angle between the polygon's surface normal and the direction of the light source, their respective colors and the intensity of the light source. Source
In brief, a shader is used to show how material will react to light.